Question

You ....

brush your teeth after every meal, if possible.
shouldn't
mustn't
must
should​

Answer 1

should

Step-by-step explanation:

we use should for giving some sort of advice and the above statement is a type of advice

Answer 2

You should brush your teeth after every meal, if possible

Step-by-step explanation:

If possible is a conditional statement/phrase and therefore should will follow it